Plugs are used in the cementing of wells for a variety of purposes, for example to provide an interface between the upper and/or lower ends of a cement slurry column and the mud or displacement fluids in the casing. When a calculated volume of cement slurry is displaced down the casing ahead of a displacement fluid, a displacement plug can be used to separate the top of the column from the displacement fluid. After substantially all of the slurry has been pumped into the annulus between the casing and the well bore wall a surface indication is needed. When a cement displacement plug bumps a float collar near the lower end of the casing, most all of the slurry will have been displaced into the annulus outside the casing.
As noted above, a typical procedure is to position the displacement plug at the top of the slurry column and then pump the plug downward with the column ahead of it. On occasion, a plug also can be run ahead of the column. When a top displacement plug is stopped by the float collar, it will have wiped the interior of the casing clean as it passed downward.
A high degree of skill and occasional guesswork is required during a well cementing operation to determine when a displacement plug has reached a certain downhole location in the casing. Various devices have been used, such as a simple pipe nipple with an internal rubber sleeve that provides a reduced diameter in the bore, in an effort to cause an observable pump pressure surge to occur at the surface when the plug passes through this bore. One or more improvements over such devices are disclosed and claimed in my U.S. Pat. No. 4,907,649 issued Mar. 13, 1990 where corrugated and fingered sleeves are used to momentarily restrict downward movement of a plug. Although this approach represents a distinct advance in the art, there remains a continuing need for a simple and reliable restriction sub that provides a distinct and positive surface indication of the point in time when a displacement plug reaches a certain point in casing. Such an indication would prevent overdisplacement of the cement, and attendant difficulties.
The general object of the present invention is to provide a new and improved restriction sub apparatus which impedes the passage of a cement displacement plug therethrough in a manner such that a positive surface indication is given.